Summary

• Fixes the P1 pipe-stage ? misparse: x |> f? parsed as x |> (f?) because |> was an ordinary Pratt binary operator and the ? postfix bound inside the right operand. §5.7 and §16 (pipe_stage) mandate (x |> f)? — a stage's trailing ? wraps the accumulated pipe application. The RHS of |> now goes through a dedicated pipe_stage() production (stage_callee [ "(" args ")" ]: path, optional turbofish, .field chains) and the caller wraps the accumulated pipe in ErrorProp. Closure stages (x |> (|v| ...)) and turbofish on non-final stage segments error as not-yet-implemented.
• Fixes impl Trait for Type (previously "expected item" — for was never handled): impl_block() now records Option<TraitRef> in the AST and parses where clauses.
• Fixes trait headers: trait Convert<T>, trait Loggable: Printable, and where clauses now parse; supertrait bounds are parsed structurally (trait_ref | lifetime, +-separated) for diagnostics but not stored, consistent with the documented bootstrap posture.
• Fixes dyn Trait<Args>: Type::Dyn now carries a TraitRef { path, args } (grammar's trait_ref = type_path [ type_args ]), so &dyn Iter<Item = i64> and Box<dyn Iter<Item = i64>> parse.

Grammar-correct behavior changes (regression-guarded by tests): x |> a + b is now (x |> a) + b — a stage is only a callee, so + b applies to the finished pipe; struct literals are no longer valid pipe stages.

Stacked-PR note: this PR targets the integration branch fix/parser-correctness-base (PR #69), not main. Wave-level docs reconciliation happens on the base PR at merge time; this PR updates only the crates/AGENTS.md subset-contract lines its fixes change.

Spec follow-up notes (no spec edits here; candidates for the open spec-debt bucket, cf. #54):

1. docs/reference/operator-precedence.md lists |> at precedence 8 but doesn't state that the pipe RHS is a pipe_stage; a naive table reading suggests x |> a + b = x |> (a + b), while §16 forces (x |> a) + b. Worth an explicit sentence in the "Key Interaction" section.
2. §16 stage_callee grammatically permits turbofish on non-final segments (a.b::<T>.c) but §5.6/§5.7 define no semantics for it; the parser reports it as not-yet-implemented.
3. §16 path_expr is ("Self" | IDENT) { "::" IDENT } (no bare self), yet x |> self.helper is a natural spelling inside actor methods; the bootstrap path() accepts self leniently (pre-existing behavior, unchanged here).

Related Issue

None (review-driven; part of the parser correctness wave tracked in PR #69).

Spec Sections Affected

None changed — this PR implements §16 pipe_stage/trait_ref/impl_block/trait_def as written. See the spec follow-up notes above.

Checklist

• Code follows the Sploosh design principles (one way to do it, explicit over implicit, etc.)
• Documentation updated in relevant docs/ pages (crates/AGENTS.md subset contract)
• Tests added or updated
• All build targets still compile (if applicable)
• Spec-only PR (skip Build Targets section if checked)

Build Targets Tested

• N/A (docs/spec only) — no codegen targets exist yet; "build" = the Rust workspace.

Test Plan

• 15 new unit tests in sploosh-parser, including AST-shape assertions: pipe_stage_question_wraps_accumulated_pipe asserts the exact ErrorProp(Binary("|>", input, parse)) tree; chained a |> f? |> g? nesting; method-chain, call-args, and turbofish stages; (x |> a) + b regression guard; error-message tests for x |> 42 and x |> (v); impl/trait/dyn shape assertions.
• 2 new corpus fixtures: tests/corpus/traits_impls.sp (trait generics/supertraits/where, inherent + trait impls, Box<dyn Iter<Item = i64>>, &dyn Convert<i64>) and tests/corpus/pipes.sp (every accepted stage shape).
• cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace (32 tests) all green locally and via scripts/docker-check.ps1 -Build (Ubuntu parity).

Summary by cubic

Fixes pipe-stage parsing so x |> f? becomes (x |> f)? and adds support for impl Trait for Type, richer trait headers, and dyn Trait<Args> types.

• Bug Fixes

  • |> now parses the RHS as a pipe_stage; the trailing ? wraps the accumulated pipe. Non-callee stages are rejected; closure stages and non-final turbofish are rejected with explicit not-yet-implemented errors.
  • impl Trait for Type parses correctly; the AST records trait_ref and supports where clauses.
  • trait headers accept generics, supertrait bounds, and where clauses (bounds parsed for validation, not stored).
  • dyn Trait<Args> stores a full trait reference, enabling cases like &dyn Iter<Item = i64>.

• Migration

  • x |> a + b is now parsed as (x |> a) + b. Add parens if you relied on the old grouping.
  • Struct literals and other non-callee expressions are no longer valid pipe stages. Use a function or method stage instead.

^{Written for commit 1c100cb. Summary will update on new commits.}